School me on DC to DC chargers

[Nomad]

I have 380 watts of lithium batteries I currently charge with a 300 watts of solar power I want to add a dc to dc charger but am clueless on what works I have heard that some of them will kill alternators can you recommend one that works?

Thanks for your input...

 

[grubworm]

a quick and basic explanation is that most alternators are designed for lead acid batteries that have a generally high resistance. the alternator is designed to pack electrons into the lead acid battery at a particular voltage and current. lithium batteries have an inherent low resistance, meaning that they will pull power harder and faster than lead acid batteries which will cause the alternator to overwork, overheat and burn up. excessive heat will damage the thin insulation on the windings of the alternator and cause a short in the armature. lithium batteries can charge very fast...I have a lithium battery charger that is 14.4v, so if the battery is wanting to draw a lot of power and is limited to 12v...then it will pull more current to make up for the lower voltage. higher current means more heat which means more problems. also, if a lithium battery is pulling hard charging from an alternator and suddenly disconnects for some reason, that can cause a huge voltage spike that will damage the rectifying diodes in the alternator.

a good dc-dc charger will monitor all these parameters and make adjustments so that the alternator and battery are not damaged. there are a lot of good videos on YouTube going more in depth on this.

personally, I'm not a huge fan of using my alternator to charge a second battery...especially a lithium one...but if I had to, I would go with a good quality charger like Victron and read their recommendations. the alternator and battery are specifically designed to support the vehicle electrical needs...mainly ignition and power for onboard computers and sensors. asking the alternator to do more work now, might require a bigger alternator or using a "smart alternator" that has a variable voltage output instead of a constant output.

you said you have 380 watts of lithium batteries, but I'm guessing you mean 380 amp hours, since that is capacity. that's a decent size battery to charge and depending on how much you use it, you might need to up the size of your alternator. if you had a 100ah or smaller battery that you are topping off and maintaining, then dc-dc charger alone should be good.

verify your battery capacity and figure out roughly how much of that battery you will use while not charging and then you will have a better idea of what kind of demand you will be asking from the alternator. just keep in mind that you are dealing with 12vdc which means a lot more current and bigger wires to handle that. a hair dryer is roughly 1300watts. using 110vac, that hair dryer will pull around 12 amps and will require 14ga wire. a 1300 watt load on 12vdc will pull around 108 amps and needs a wire about as big around as your thumb.

the dc-dc charger will monitor and regulate voltage/current, but all the other components in the system need to be the correct ones in order for the charger to do its job properly

 

[K12]

Grubworm hit it on the head with the alternator. when I put my victron DC-DC into the system I ran it off the battery of the truck to the trailer. I never had issues with this and the alternator never had an issue. With that, a 2500 diesel has a more powerful alternator, and more "extra space" for power output that a smaller vehicle would. Do the research on the alternator and its output that you have, with how much power your vehcile requires for it to run properly. Adjust from there. It may require a bigger alternator.

 

[highboy4x4]

I concur with both assessments, especially about wire size. I have a F350 so my alternator has more capacity than a even a stock jeep. I use a Renogy DC-DC 50Amp charge controller and I have zero issues now. I also added a cut off switch so I do not use it all the time. Only when my 2-100 watt solar panels don't see the sun.

I am sure murphy will visit me and I will eat these words but karma be damned!

 

[Renegade]

In addition to the Victron and Renogy previously mentioned, I use a RedArc. Lots of choices out there that work.

 

[rtexpeditions]

They are better for your alternator than just directing power to a second battery via a solenoid/relay, which can result in large current flows, especially if one battery is flat.

The amps drawn from your alternator/car battery are fixed by the circuitry of the DCDC, so a 20A unit will only ever draw ~20A, a 50A unit ~50A and so forth.
I would check how much headroom you have between your alternator and the current draw of your car (not including starting), as to how much that is, but 50A is not unreasonable.

The advantages of a DCDC are that it will charge your 2nd battery even if the vehicle's volts is lower than 12v. This is important if you have a smart alternator and/or a stop/start vehicle.
They also provide a better charging profile than the alternator and can cope with different battery types (eg, Lithium) than the alternator and are similar to a mains charger. So it will provide more capacity and longer battery life.

As the current is limited, you only need cables large enough to support the current draw of the DCDC. Voltage drop can be ignored, within reason, as the DCDC will boost that back up to charging voltage.

You can also use most DCDC chargers as an MPPT solar regulator/charger if you connect solar panels.

If your vehicle has a smart alternator, make sure you have the right DCDC, as it will require a lower input voltage and trigger circuit from an "ignition on" wire.

Always connect the DCDC -ve to the vehicle chassis, and not the battery negative terminal, so the current is monitored by the vehicle's shunt.

I use a Redarc BCDC40-LV. It is an old model, and was a prototype from Redarc. No fancy monitoring, it just works. Solar input is via a relay that switches in when the ignition is off. I've been running it continuously for over a year now.

 

[traildog]

I’ve had the Redqrc BCDC1225D for over 6 years without issue. I have it in my 4Runner and has been used to charge a second AGM battery and a lithium battery. I switched to the lithium after I lost the space for the AGM due to a supercharger upgrade.